Use of computational fluid dynamics in the design of bubble column reactors

Investigations into the modelling techniques that depict the transport of discrete phases (gas bubbles or solid particles) and model biochemical reactions in a bubble column reactor are discussed here. The mixture model was used to calculate gas-liquid, solid-liquid and gasliquid-solid interactions. Multiphase flow is a difficult phenomenon to capture, particularly in bubble columns where the major driving force is caused by the injection of gas bubbles. The gas bubbles cause a large density difference to occur that results in transient multi-dimensional fluid motion. Standard design procedures do not account for the transient motion, due to the simplifying assumptions of steady plug flow. Computational fluid dynamics (CFD) can assist in expanding the understanding of complex flows in bubble columns by characterising the flow phenomena for many geometrical configurations. Therefore, CFD has a role in the education of chemical and biochemical engineers, providing the examples of flow phenomena that many engineers may not experience, even through experimentation. The performance of the mixture model was investigated for three domains (plane, rectangular and cylindrical) and three flow models (laminar, k-e turbulence and the Reynolds stresses). mThis investigation raised many questions about how gas-liquid interactions are captured numerically. To answer some of these questions the analogy between thermal convection in a cavity and gas-liquid flow in bubble columns was invoked. This involved modelling the buoyant motion of air in a narrow cavity for a number of turbulence schemes. The difference in density was caused by a temperature gradient that acted across the width of the cavity. Multiple vortices were obtained when the Reynolds stresses were utilised with the addition of a basic flow profile after each time step. To implement the three-phase models an alternative mixture model was developed and compared against a commercially available mixture model for three turbulence schemes. The scheme where just the Reynolds stresses model was employed, predicted the transient motion of the fluids quite well for both mixture models. Solid-liquid and then alternative formulations of gas-liquid-solid model were compared against one another. The alternative form of the mixture model was found to perform particularly well for both gas and solid phase transport when calculating two and three-phase flow. The improvement in the solutions obtained was a result of the inclusion of the Reynolds stresses model and differences in the mixture models employed. The differences between the alternative mixture models were found in the volume fraction equation (flux and deviatoric stress tensor terms) and the viscosity formulation for the mixture phase.

Gas-liquid simulation of an airlift bubble column reactor

The simulation of two-phase flow for an experimental airlift reactor (32-l volume) using commercially available software from Fluent Incorporated is presented here (http://www.fluent.co.uk). Data from the simulation is compared with the experimental data obtained by the tracking of a magnetic particle and analysis of the pressure drop to determine the gas hold-up. Comparisons between vertical velocity and gas hold-up were made for a series of experiments where the superficial gas velocity in the riser was adjusted between 0.01 and 0.075 m s-1. © 2003 Elsevier B.V. All rights reserved.

Gas-liquid-solid flow modelling in a bubble column

An alternative approach to the modelling of solid-liquid and gas-liquid-solid flows for a 5:1 height to width aspect ratio bubble column is presented here. A modified transport equation for the volume fraction of a dispersed phase has been developed for the investigation of turbulent buoyancy driven flows (Chem. Eng. Proc., in press). In this study, a modified transport equation has been employed for discrete phase motion considering both solid-liquid and gas-liquid-solid flows. The modelling of the three-phase flow in a bubble column was achieved in the following case: injecting a slug of solid particles into the column for 10 s at a velocity of 0.1 m s-1 and then the gas phase flow was initiated with a superficial gas velocity of 0.02 cm s-1. © 2003 Elsevier B.V. All rights reserved.

The modelling of buoyancy driven flow in bubble columns

Using the analogy between lateral convection of heat and the two-phase flow in bubble columns, alternative turbulence modelling methods were analysed. The k-ε turbulence and Reynolds stress models were used to predict the buoyant motion of fluids where a density difference arises due to the introduction of heat or a discrete phase. A large height to width aspect ratio cavity was employed in the transport of heat and it was shown that the Reynolds stress model with the use of velocity profiles including the laminar flow solution resulted in turbulent vortices developing. The turbulence models were then applied to the simulation of gas-liquid flow for a 5:1 height to width aspect ratio bubble column. In the case of a gas superficial velocity of 0.02 m s-1 it was determined that employing the Reynolds stress model yielded the most realistic simulation results. © 2003 Elsevier B.V. All rights reserved.

Three-dimensional gas-liquid simulation of an airlift bubble column reactor

Basic hydrodynamic parameters of an airlift reactor with internal loop were estimated experimentally and simulated using commercially available CFD software from Fluent. Circulation velocity in a 32-dm(3)-airlift reactor was measured using the magnetic tracer method, meanwhile the gas hold-up was obtained by analysis of the pressure drop using the method of inverted U-tube manometers. Comparison of simulated (in two and three dimensions) and experimental data was performed at different superficial gas velocities in the riser.

CFD and bubble column reactors:simulation and experiment

The simulation of two-phase flow in bubble columns using commercially available software fromFluent Incorporated is presented here. Data from a bubble column with a ratio of height to thecolumn diameter of 5 : 1 are compared with simulations and experimental results for time-averaged velocity and Reynolds stress proles are used to validate transient, two-dimensional simulations.The models are based on multiphase biological reactors with applications in the food industry. An example case of the mass transfer of oxygen through the liquid phase is also presented.

Buborékok és legendák. Válságok és válságmagyarázatok - a tulipánmánia és a Déltengeri Társaság, I. rész (Bubbles and myths, crises and explanations: tulip mania and the South Sea bubble. I.)

A 2007 óta kibontakozó válság újból divatba hozta a korai buborékok és válságok témáját. A jelenlegi válság nyomán újraéledtek a makroökónómiai viták, kétségessé vált a "nagy mérséklődés" és az újklasszikus szintézis érvényessége. Az 1634-1637 közötti holland tulipánmánia - mint az első buborék - sokféle értelmezése ismeretes: a tömeghisztéria kitörésétől a hatékony pénzügyi piacok korai példáján át a kulturális sokkig. Áttekintve ezeket, a cikk visszavezeti a tulipánmánia közismert anekdotikus leírásait az eredeti forrásokig, és megmutatja, milyen módon és céllal használták fel ezeket a közgazdászok és történészek saját elemzéseikben. / === / The accounts of early bubbles and crises are becoming fashionable again in economic discourse during the recent downturn. The article, having looked at the revival of macroeconomic debate provoked by the failure of current theory, sums up various interpretations of the Dutch tulip mania of 1634-7. These range from an outburst of popular madness, through an early example of an efficient financial market, to an instance of culture shock. Some well-known anecdotes about tulip mania are traced back to their original sources, and the article explores the various patterns and intentions in the use economists and historians have made of them.

Buborékok és legendák. Válságok és válságmagyarázatok - II/1. rész. A Déltengeri Társaság (Bubbles and myths crises and explanations II/1.: the South Sea bubble)

A cikk a tulipánmániával foglalkozó tanulmány folytatása (Madarász [2009]). A Déltengeri Társaság 1720-as fellendülése és kipukkadása mindmáig egyike a pénzügyi történelem leghíresebb és leggyakrabban emlegetett buborékainak. A közgazdaságtanban a buborék sokáig nem volt fontos téma, de az utóbbi időben ismét divatba jött. A tanulmány először a buborékmetafora néhány irodalmi példáját mutatja be, majd összefoglalja az angol államadósság kialakulását, a korai modern fiskális-militarista állam létrejöttét és a pénzügyi forradalom különböző interpretációit. A Déltengeri Társaság történetének, az adósság-részvény csere lebonyolításának és a korabeli vélemények spektrumának ismertetése után áttekintést ad arról, milyen módon és céllal használták fel az 1720-as eseményeket közgazdászok és történészek saját magyarázataikban. Ezek skálája a tudatos csalástól a befektetők irracionális mániáján át a racionális buborék kialakulásáig terjed. / === / The first part of the study (published here in 2009) was devoted to "tulipmania". This article continues the account of early bubbles and crises with the 1720 boom and bust of the South Sea Company, which is to this day one of the best known and most cited examples in history. For several decades, bubbles were not seen as an important issue in economic and financial theory, but recent events have focused attention on them again. The author introduces some historical examples of the bubble metaphor in literature, before giving an account of the emergence of British public debt and the fiscal-military state, and summarizing various interpretations of the financial revolution. An account of the South Sea Bubble, a detailed description of the debt-equity swap, and citations from some contemporary investors and observers are followed by an overview of the way the events of 1720 were used subsequently by various economists and historians in their own theorizing and explanations. The interpretations placed on it range from deliberate fraud, through irrational investment mania, to the emergence of a rational bubble.

Buborékok és legendák. Válságok és válságmagyarázatok II/2. rész. A Déltengeri Társaság (Bubbles and myths crises and explanations II/2.: the South Sea bubble)

A cikk a tulipánmániával foglalkozó tanulmány folytatása (Madarász [2009]). A Déltengeri Társaság 1720-as fellendülése és kipukkadása mindmáig egyike a pénzügyi történelem leghíresebb és leggyakrabban emlegetett buborékainak. A közgazdaságtanban a buborék sokáig nem volt fontos téma, de az utóbbi időben ismét divatba jött. A tanulmány először a buborékmetafora néhány irodalmi példáját mutatja be, majd összefoglalja az angol államadósság kialakulását, a korai modern fiskális-militarista állam létrejöttét és a pénzügyi forradalom különböző interpretációit. A Déltengeri Társaság történetének, az adósság-részvény csere lebonyolításának és a korabeli vélemények spektrumának ismertetése után áttekintést ad arról, milyen módon és céllal használták fel az 1720-as eseményeket közgazdászok és történészek saját magyarázataikban. Ezek skálája a tudatos csalástól a befektetők irracionális mániáján át a racionális buborék kialakulásáig terjed. / === / The first part of the study (published here in 2009) was devoted to "tulip-mania". This article continues the account of early bubbles and crises with the 1720 boom and bust of the South Sea Company, which remains to this day one of the best known and most cited examples in history. For several decades, bubbles were not seen as an important issue in economic and financial theory, but recent events have focused attention on them again. The author introduces some historical examples of the bubble metaphor in literature, before describing the emergence of British public debt and the fiscal/military state, and summarizing various interpretations of the financial revolution. An account of the South Sea Bubble, a detailed description of the debt-equity swap, and citations from some contemporary investors and observers are followed by an overview of how the events of 1720 were used subsequently by various economists and historians in their theorizing and explanations. Such interpretations range from deliberate fraud, through irrational investment mania, to the emergence of a rational bubble.

Pore-water concentration of chloride in sediment cores and equilibrium temperatures and gas bubble analysis at ROV stations from the Vodyanitskii mud volcano

Vodyanitskii mud volcano is located at a depth of about 2070 m in the Sorokin Trough, Black sea. It is a 500-m wide and 20-m high cone surrounded by a depression, which is typical of many mud volcanoes in the Black Sea. 75 kHz sidescan sonar show different generations of mud flows that include mud breccia, authigenic carbonates, and gas hydrates that were sampled by gravity coring. The fluids that flow through or erupt with the mud are enriched in chloride (up to 650 mmol L**-1 at 150-cm sediment depth) suggesting a deep source, which is similar to the fluids of the close-by Dvurechenskii mud volcano. Direct observation with the remotely operated vehicle Quest revealed gas bubbles emanating at two distinct sites at the crest of the mud volcano, which confirms earlier observations of bubble-induced hydroacoustic anomalies in echosounder records. The sediments at the main bubble emission site show a thermal anomaly with temperatures at 60 cm sediment depth that were 0.9 °C warmer than the bottom water. Chemical and isotopic analyses of the emanated gas revealed that it consisted primarily of methane (99.8%) and was of microbial origin (dD-CH4 = -170.8 per mil (SMOW), d13C-CH4 = -61.0 per mil (V-PDB), d13C-C2H6 = -44.0 per mil (V-PDB)). The gas flux was estimated using the video observations of the ROV. Assuming that the flux is constant with time, about 0.9 ± 0.5 x 10**6 mol of methane is released every year. This value is of the same order-of-magnitude as reported fluxes of dissolved methane released with pore water at other mud volcanoes. This suggests that bubble emanation is a significant pathway transporting methane from the sediments into the water column.

Lily with bubble gum

General note: Title and date provided by Bettye Lane.

Bubble Growth From First Principals

We consider the simplest relevant problem in the foaming of molten plastics, the growth of a single bubble in a sea of highly viscous Newtonian fluid, and without interference from other bubbles. This simplest problem has defied accurate solution from first principles. Despite plenty of research on foaming, classical approaches from first principles have neglected the temperature rise in the surrounding fluid, and we find that this oversimplification greatly accelerates bubble growth prediction. We use a transport phenomena approach to analyze the growth of a solitary bubble, expanding under its own pressure. We consider a bubble of ideal gas growing without the accelerating contribution from mass transfer into the bubble. We explore the roles of viscous forces, fluid inertia, and viscous dissipation. We find that bubble growth depends upon the nucleus radius and nucleus pressure. We begin with a detailed examination of the classical approaches (thermodynamics without viscous heating). Our failure to fit experimental data with these classical approaches, sets up the second part of our paper, a novel exploration of the essential decelerating role of viscous heating. We explore both isothermal and adiabatic bubble expansion, and also the decelerating role of surface tension. The adiabatic analysis accounts for the slight deceleration due to the cooling of the expanding gas, which depends on gas polyatomicity. We also explore the pressure profile, and the components of the extra stress tensor, in the fluid surrounding the growing bubble. These stresses can eventually be frozen into foamed plastics. We find that our new theory compares well with measured bubble behavior.

Estimating abundance and spatial distribution patterns of the bubble crab Dotilla fenestrata (Crustacea : Brachyura)

The bubble crab Dotilla fenestrata forms very dense populations on the sand flats of the eastern coast of Inhaca Island, Mozambique, making it an interesting biological model to examine spatial distribution patterns and test the relative efficiency of common sampling methods. Due to its apparent ecological importance within the sandy intertidal community, understanding the factors ruling the dynamics of Dotilla populations is also a key issue. In this study, different techniques of estimating crab density are described, and the trends of spatial distribution of the different population categories are shown. The studied populations are arranged in discrete patches located at the well-drained crests of nearly parallel mega sand ripples. For a given sample size, there was an obvious gain in precision by using a stratified random sampling technique, considering discrete patches as strata, compared to the simple random design. Density average and variance differed considerably among patches since juveniles and ovigerous females were found clumped, with higher densities at the lower and upper shore levels, respectively. Burrow counting was found to be an adequate method for large-scale sampling, although consistently underestimating actual crab density by nearly half. Regression analyses suggested that crabs smaller than 2.9 mm carapace width tend to be undetected in visual burrow counts. A visual survey of sampling plots over several patches of a large Dotilla population showed that crab density varied in an interesting oscillating pattern, apparently following the topography of the sand flat. Patches extending to the lower shore contained higher densities than those mostly covering the higher shore. Within-patch density variability also pointed to the same trend, but the density increment towards the lowest shore level varied greatly among the patches compared.

EFFECTS OF BUBBLE-FACILITATED VOC TRANSPORT FROM LNAPL SMEAR ZONES ON VAPOR INTRUSION

Light non-aqueous phase liquid (LNAPL) sources can pose a significant threat to indoor air through vapour intrusion (VI). Most conceptual and numerical models of VI assume that the transport of volatile organic compounds (VOCs) is a diffusion-limited process. Recently, alternate conditions have been identified that could lead to faster transport, including the presence of preferential pathways and methanogenic gas production. In this study, an additional mechanism that could lead to faster transport was investigated: bubble-facilitated VOC transport from LNAPL smear zones. A laboratory investigation was preformed using pentane in one-dimensional laboratory columns and two-dimensional visualization experiments. Results of the column experiments showed that average VOC mass fluxes in the bubble-facilitated columns were over two orders of magnitude greater than in the diffusion-limited columns. In addition, the flux signal was intermittent, consistent with expectations of bubble-facilitated transport as bubbles expand, mobilize and are released to the vadose zone at various times during the test. The results from the visualization experiments showed gas fingers growing and mobilizing over time, which supports the findings of the column experiments. In conclusion, these results demonstrate the potential for bubble-facilitated VOC transport to affect mass transfer in LNAPL smear zones, and lead to increased indoor air concentrations by VI.